The bolt holes in the block where the starter mounts will indicate which flywheel you can use. If the holes are in a straight line relative to the back of the block the 153 tooth wheel and starter must be used. Starter holes in the block staggered from each other will allow the 168 tooth wheel and starter. Some blocks have all 3 starter holes but they are rare.
Additionally the bellhousing that is flat down the starter side will only accommodate the 153 wheel & starter combo. Bellhousing with a bulge on the side for the starter nose is for the larger wheel set up. It is possible to drill and tap the bottom of the block for the needed starter bolt hole to match your flywheel size and starter mount. The outer bolt hole is the same for both. The inner hole closest to the crank is the one that differs. If you do drill the bottom of the block be absolutely sure of its position or starter drive and ring gear problems will occur.
